Building a Flow bench, was: Measuring air flow

[Bernd Felsche]

Kris  Weldy tapped away at the keyboard with:
> From: "Bernd Felsche" <bernie at innovative.iinet.net.au>
> > If you're looking at comparable flow rates, you need a powerful
> > motor and a comparably-efficient means of moving air. Back in 1979,
> > I built a flow rig that used a large Rootes blower off a Nissan
> > truck, driven by a 10hp electric motor. A 200-litre drum was used as
> > a (tunable) flow damper.

> What would that power be -compared to a roots blower,one uses
> torque supplied from the motor ,the other uses expanding exhaust
> gases.. 

And the _cooling_ of the gases. The thermal gradient across the
turbine is very significant.

In the case of the test rig, the blower was used to draw air through
the test rig; not to blow it. There were a number of reasons for
that; one that comes to mind is that there would be no heat of
compression in the _test_ flow. Another was that it made for
more-convenient instrumentation (manometers open to atmosphere).

It's fairly straight-forward to calculate the minimum amount of
power required based on the nominal flow rate and the diameter of
the meter (as well as the density of air). 

Typical peak flow meter velocities are around 60 m/s (IIRC).  For an
80mm diameter meter that gives about 0.3 m^3/s. (btw that is about
what a "5-litre" engine ideally flows at 8000 rpm)

Air at that velocity has a K.E. of about 2.2 kJ/m^3. So the minimum
amount of drive power required will be 650 W; with perfect blower or
compressor and no flow losses. Allow for inefficiencies and your
drive power, wherever it comes from, will have to be much greater.

-- 
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